Optical heads produce a focused beam or light on a medium containing information and detect the light reflected from the medium to determine the information content of the medium. Mechanisms for maintaining the focus and tracking of the optical head are required. With the recent advances in semiconductor lasers, there has been an increasing use of these lasers in data retrieval and recording systems. The compact audio disc player is a significant example of how lasers are used in playing back prerecorded music, which is a form of information. The concept of the compact audio disc player can be applied to the storage of data for a large computer network, mini computers or even personal computers.
When lasers are employed in these devices, the light emitted by the lasers must be controlled by appropriate optical components to produce a very small spot of light on the medium surface. Light reflected off of the medium is projected back to a detector from which recorded information and other signals relating to the status of the focus and tracking can be derived. Some examples of patents covering optical systems for such applications are U.S. Pat. Nos. 3,962,720, 3,969,573, 4,034,403, 4,057,833, 4,135,083, 4,143,402, 4,193,091, 4,198,657, 4,458,980, 4,486,791, 4,779,943, and West German Pat. No. 2501124.
FIG. 1 shows a prior art optical head from U.S. Pat. No. 4,731,772 that uses a hologram lens for both beam splitting and focus detection functions. An optical head 10 consists of a laser pen 12 and a focusing and tracking actuator 14. A laser beam 16 is focused on a grooved information medium 18 at a spot 20. Laser pen 12 consists of a semiconductor laser and detector 22, a collimating lens 24, and a hologram lens 26. The focus and tracking actuator consists of an objective lens 28 that can be moved up and down by a magnetic coil 29 for focusing the laser beam.
FIG. 2 shows a front view of the semiconductor laser and photodetector 22. A semiconductor laser 30 is mounted on a heatsink 32. A four quadrant photodetector 34 is mounted on the face of heatsink 32. A photodetector 36 is located behind semiconductor laser 30 to measure the light emitted from the semiconductor laser. Photodetector 36 is at an angle so that it does not reflect light back into the semiconductor laser. In operation, laser beam 16 is emitted by semiconductor laser 30 and is collimated or made parallel by collimating lens 24. This collimated beam passes through hologram lens 26 to produce a zero order diffracted beam and a number of higher order diffracted beams. The zero order diffracted beam continues on the same path, not at an angle, and is the only beam used in the forward light path of the optical head. This beam is focused on medium 18 by objective lens 28 which can be moved with magnetic coil 29.
On the return path, the reflected laser beam again hits holograms lens 26 producing zero and higher order diffracted beams. The zero order beam returns to the semiconductor laser and is not used for detection. One of the higher order beams, generally the first order beam, is imaged onto the photodetector by the combination of hologram lens 26 and collimating lens 24. The hologram lens not only diffracts the returned beam toward the four-quadrant detector 34, but also acts like a cylindrical lens to produce a focusing and tracking pattern on four-quadrant photodetector 34 which varies according to the focus and tracking of spot 20.
Examples of the focusing pattern on four quadrant detector 34 are shown in FIGS. 3A-3C, with the best being shown in FIG. 3B. A focus error signal is produced by (A+C)-(B+D). FIG. 3A shows the focusing pattern when the beam is out of focus because the medium is too close to the objective lens. FIG. 3C shows the focusing pattern when the beam is out of focus due to the medium being too far from the objective lens. FIG. 3B shows the focusing pattern (called the circle of least confusion) when the beam is in focus.
FIG. 3B also shows overlapping beams 40 and 42 which are produced by the grating effect of the grooved structure of medium 18. A tracking signal or tracking push-pull signal is given by (A+D)-(B+C). The beam will be on track and the tracking signal will be equal to zero when beams 40 and 42 are of equal brightness. One of the disadvantages of the push-pull tracking error signal is that it can be affected by large motion of the objective lens relative to the collimating lens. One method to avoid this difficulty is to have the complete optical head follow the motion of the focus and tracking actuator.
FIGS. 1-3 show an optical head using a holographic lens for both beam splitting and focus error detection. In that system the focus error and the tracking error are derived from the same four segment detectors. Because all the information related to the data signal and servo signals is derived from a single beam focused on the information medium, this type of optical head is often called a single beam optical head. A more popular optical head used in many commercial products is called a three beam optical head which derives the necessary information by focusing three laser beams onto the information medium.
FIG. 4 shows one embodiment of a prior art three beam optical head 50. The optical head consists of a laser pen 52 and a focusing and tracking actuator 54. A laser beam 56 is focused on an information medium 58 at a spot 60. Laser beam 56 is emitted from a semiconductor laser 62 in the shape of an elliptical cone. It is known to correct some aberrations in the laser beam such as astigmatism by placing a tilted glass cap onto the front surface of the semiconductor laser. Laser beam 56 first passes through a grating 64 which diffracts the laser beam into three beams 56, 66, and 67 as shown in FIG. 5. Of course, each of the laser beams originates from semiconductor laser 62. However, the diffracted beams 66 and 67 appear to originate from virtual laser sources 68 and 69, respectively. The angle of separation between each of the diffracted beams and the incident beam is small. For the sake of simplicity in this discussion, the beam indicated as 56 is assumed to include the two diffracted beams 66 and 67. Beam 56 passes unchanged through a beam splitter 70 to a collimating lens 72. The beam is collimated or made substantially parallel by the collimating lens. The parallel beam then impinges upon an objective lens 74 which focuses beam 56 onto medium 58 at spot 60. The focusing of lens 74 is accomplished through the use of a magnetic coil 76 which moves objective lens 74 up and down with respect to the medium 58. In addition a tracking coil may move objective lens 74 horizontally. The three beams are reflected off reflective medium 58 through a beam splitter 70. Part of each of the three beams are reflected by the beam-splitter, pass through a cylindrical lens 77 to a six segment photodetector 78.
The three beams focused on the medium are shown in FIGS. 6A, 6B, and 6C in three different positions 56a, 66a and 67a (corresponding to beams 56, 66 and 67) with respect to a data track 86 between land areas 87. FIG. 6A shows center spot 56a, which reads the information from the medium, off to the left of track 86. In this case side spot 66a is on data track 86 and side spot 67a is on land area 87. The tracks have lower reflectivity than the land areas. As a result the beam reflected from side spot 66a has a lower intensity than the beam reflected from side spot 67a. In FIG. 6B, center spot 56a is exactly on track. Side spots 66a and 67a symmetrically straddle the data track and the land areas. The beams reflected from side spots 66a and 67a have the same intensity. FIG. 6C shows center spot 56a to the right of the data track. In this case the beam reflected from side spot 66a has greater intensity than the beam reflected from side spot 67a. The difference between the amount of light reflected from the two side spots produces the tracking error signal.
FIG. 7 shows the three beams imaged at spots 56b, 66b, and 67b on six segment photodetector 78 as described in FIG. 4 above. Middle beam 56 of the three beams is imaged at center spot 56b on quadrant detector 95. Side beams 66 and 67 are imaged at side spots 66b and 67b on photodetectors 96 and 97, respectively. The quadrant detector is used for focusing as described in FIGS. 3A-C above. However, photodetectors 96 and 97 are used for tracking by detecting the difference between the intensities of beams 66 and 67 as described with reference to FIGS. 6A-6C above. This provides for greater sensitivity of tracking than single beam optical heads.